Priority is claimed to Korean Patent Application No. 2003-94684, filed on Dec. 22, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a flip-chip nitride light emitting device and a method of manufacturing thereof, and more particularly, to a flip-chip nitride light emitting device having an electrode structure capable of improving light emitting efficiency and a method of manufacturing thereof.
2. Description of the Related Art
In technology for manufacturing light emitting devices such as light emitting diodes and laser diodes by using nitride compound semiconductors such as gallium nitride semiconductors, ohmic contact structures between the semiconductors and electrodes are very important. Currently, commercially-available gallium nitride light emitting devices are formed on insulating sapphire (Al2O3) substrates.
These gallium nitride light emitting devices are classified into top-emitting light emitting devices (TLEDs) and flip-chip light emitting devices (FCLEDs).
The top-emitting light emitting devices emit light through an ohmic electrode layer in contact with a p type clad layer.
The top-emitting light emitting devices have poor electrical characteristics such as a low current injection and current spreading, which result from the thin film characteristics of the p type clad layer that has a low concentration of holes. These poor electrical characteristics can be overcome by using an ohmic contact electrode having a high transmittance and a low sheet resistance.
In general, the top-emitting light emitting devices have a metal thin film structure which mainly contains a transition metal such as nickel (Ni). In particular, oxidized semi-transparent metal thin films of nickel (Ni)/gold (Au) have been widely used for the top-emitting light emitting devices.
It is disclosed that, if a metal thin film mainly containing nickel (Ni) is subject to a thermal annealing process in oxygen (O2) ambience, a semi-transparent ohmic contact layer having a non-contact resistance of 10−3 to 10−4 Ω·cm2 can be obtained.
Such a low non-contact resistance results in the decrease in Schottky barrier height (HBT). As a result, when being thermally annealed under an oxygen atmosphere (O2) at a temperature ranging from 500 to 600° C., major carriers, that is, holes can be easily supplied in a vicinity of a surface of the gallium nitride, so that an effective concentration in a vicinity of the surface of the gallium nitride can increase. On the other hand, if the stacked structure of nickel (Ni)/gold (Au) is in contact with the p type gallium nitride and subject to an thermal annealing process, complexes between magnesium (Mg) and hydrogen (H) can be removed and reactivation phenomenon occurs, so that the concentration of the dopant of magnesium (Mg) on the surface of gallium nitride. As a result, the effective concentration of carriers on the surface of gallium nitride becomes 1019 or more and tunneling conduction on the interface between the gallium nitride and the electrode (nickel oxide) occurs, so that the ohmic conduction characteristics can be obtained.
However, there is a problem in that the top-emitting light emitting devices implemented by using the semi-transparent electrode thin film of nickel (Ni)/gold (Au) has too low light emitting efficiency to implement a large-capacity high-brightness light emitting device.
Recently, in order to implement a large-capacity high-brightness light emitting device, there have been demands for developing flip-chip light emitting devices implemented by using one of silver (Ag), silver oxide (Ag2O) and aluminum (Al), which are popularly used as a material of a highly reflective layer.
Although these metals having a high reflectance temporarily provides a high light emitting efficiency, it is difficult to form a low non-contact resistance ohmic contact due to their low work function, so that the life time of the flip-chip light emitting devices may be shortened and contact characteristics thereof associated with the gallium nitride can be deteriorated. As a result, there is a problem in that the flip-chip light emitting devices cannot provide a good contact characteristic and durability.
More specifically, since aluminum (Al) metal having a low work function and forming nitride during a thermal annealing process has a tendency to form a Schottky contact other than an ohmic contact at the interface of the p type gallium nitride, the aluminum (Al) metal barely can be used. Unlike aluminum (Al) metal, silver (Ag) metal forms the ohmic contact with the gallium nitride. However, since silver (Ag) metal has a poor mechanical adhesion to the gallium nitride and thermal instability, there is a problem in that it is difficult to ensure reliability of manufacturing and operation the light emitting devices.
In order to solve the problems, ohmic contact layers having a low non-contact resistance and a high reflectance have been actively researched and developed.
Mensz et al. proposed a two-layered structure of nickel (Ni)/aluminum (AL) and nickel (Ni)/silver (Ag) in an article (Electronics Letters 33 (24) pp. 2066). However, since this two-layered structure cannot easily form the ohmic contact, there is a problem in that a large amount of heat is generated due to a high operating voltage of the light emitting diode.
Recently, Michael R. Krames et al. disclosed an electrode structure of nickel (Ni)/silver (Ag) and gold (Au)/nickel oxide (NiOx)/aluminum (Al) in U.S. Patent Publication No. US 2002/0171087 A1. However, there is a problem in that the electrode structure has a poor adhesion and light emitting efficiency is lowered due to its diffused reflection.